linux-kernel - Re: [PATCH] arch/powerpc/include/asm/barrier.h: redefine rmb and wmb to lwsync

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Message-ID: <Y/XPZl8V4/0Bjegy@li-a450e7cc-27df-11b2-a85c-b5a9ac31e8ef.ibm.com>
Date:   Wed, 22 Feb 2023 13:46:38 +0530
From:   Kautuk Consul <kconsul@...ux.vnet.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch/powerpc/include/asm/barrier.h: redefine rmb and wmb
 to lwsync

On Wed, Feb 22, 2023 at 07:02:34AM +0000, Christophe Leroy wrote:
> 
> 
> Le 22/02/2023 à 07:01, Kautuk Consul a écrit :
> > A link from ibm.com states:
> > "Ensures that all instructions preceding the call to __lwsync
> >   complete before any subsequent store instructions can be executed
> >   on the processor that executed the function. Also, it ensures that
> >   all load instructions preceding the call to __lwsync complete before
> >   any subsequent load instructions can be executed on the processor
> >   that executed the function. This allows you to synchronize between
> >   multiple processors with minimal performance impact, as __lwsync
> >   does not wait for confirmation from each processor."
> > 
> > Thats why smp_rmb() and smp_wmb() are defined to lwsync.
> > But this same understanding applies to parallel pipeline
> > execution on each PowerPC processor.
> > So, use the lwsync instruction for rmb() and wmb() on the PPC
> > architectures that support it.
> > 
> > Also removed some useless spaces.
> > 
> > Signed-off-by: Kautuk Consul <kconsul@...ux.vnet.ibm.com>
> > ---
> >   arch/powerpc/include/asm/barrier.h | 12 +++++++++---
> >   1 file changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> > index e80b2c0e9315..553f5a5d20bd 100644
> > --- a/arch/powerpc/include/asm/barrier.h
> > +++ b/arch/powerpc/include/asm/barrier.h
> > @@ -41,11 +41,17 @@
> >   
> >   /* The sub-arch has lwsync */
> >   #if defined(CONFIG_PPC64) || defined(CONFIG_PPC_E500MC)
> > -#    define SMPWMB      LWSYNC
> 
> This line shouldn't be changed by your patch
I mentioned it in the commit message.
But if you want I'll remove this. Did this because the rest
of the file doesn't have these spaces.
> 
> > +#undef rmb
> > +#undef wmb
> 
> I see no point with defining something and undefining them a few lines 
> later.
> 
> Instead, why not do:
> 
> #define mb()   __asm__ __volatile__ ("sync" : : : "memory")
> 
> #if defined(CONFIG_PPC64) || defined(CONFIG_PPC_E500MC)
> #define rmb()	({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> #define wmb()	({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> #else
> #define rmb()  __asm__ __volatile__ ("sync" : : : "memory")
> #define wmb()  __asm__ __volatile__ ("sync" : : : "memory")
> #endif
> 
I thought of doing that earlier, but there exists one more #elif
for CONFIG_BOOKE and then the #else.
That way we would have to put 3 different #defines for rmb and wmb
and I wanted to avoid that.
> By the way, why put it inside a ({ }) ?
checkpatch.pl asks for ({}).
> And I think nowdays we use "asm volatile" not "__asm__ __volatile__"
I was just following what was there in the file already.
Can change this if required.
> 
> Shouldn't you also consider the same for mb() ?
My change wasn't meant to address newer usages of as volatile
#defines. I just wanted to redefine the rmb and wmb #defines
to lwsync.
> 
> Note that your series will conflict with b6e259297a6b ("powerpc/kcsan: 
> Memory barriers semantics") in powerpc/next tree.
I thought of defining the __rmb and __wmb macros but I decided against
it because I didn't understand kcsan completely.
I used the standard Linus' tree, not powerpc/next.
Can you tell me which branch or git repo I should make this patch on ?
> 
> > +/* Redefine rmb() to lwsync. */
> 
> WHat's the added value of this comment ? Isn't it obvious in the line 
> below that rmb() is being defined to lwsync ? Please avoid useless comments.
Sure.
> 
> > +#define rmb()	({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> > +/* Redefine wmb() to lwsync. */
> > +#define wmb()	({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> > +#define SMPWMB      LWSYNC
> >   #elif defined(CONFIG_BOOKE)
> > -#    define SMPWMB      mbar
> 
> This line shouldn't be changed by your patch
> 
> > +#define SMPWMB      mbar
> >   #else
> > -#    define SMPWMB      eieio
Ok. Can change my patch.
> 
> This line shouldn't be changed by your patch
> 
> > +#define SMPWMB      eieio
> >   #endif
Sure. Will retain this too.
> >   
> >   /* clang defines this macro for a builtin, which will not work with runtime patching */